The invention presented herein relates to contact fixing stations for an imaging apparatus wherein the image is formed by toner particles carried on a receptor sheet which are permanently adhered to the receptor sheet at a fixing station of the apparatus and more particularly to the apparatus for removing toner material that is offset to a rotating fixing roller of the fixing station and for applying offset preventive liquid to the fixing roller.
The use of toner particles for forming images on a receptor sheet in various image forming machines, such as office copiers and the like, requires a fixing station within the machine for permanently adhering the toner particles to the receptor sheets. Apparatus for fixing stations which use contact with the toner particles to permanently affix the toner material to the receptor sheet utilize a fixing roller, which contacts the toner, and a back-up roller, at least one of which is driven, to move the receptor sheet between the rollers. One approach relies on the application of heat to the toner material via the fixing roller which is maintained at a temperature sufficient to elevate the temperature of the toner material to a point where the constituents of the toner material coalesce and become tacky. This action causes the toner material to be absorbed to some extent into the fibers of the receptor sheet material which, in many instances, constitutes plain paper. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be permanently bonded to the receptor sheet. By controlling the heat transfer to the toner, virtually no offset of the toner particles from the receptor sheet to the fixing roller is experienced under normal conditions. By careful control of the heat transferred to the toner material, the heat applied to the surface of the fixing roller will not raise the temperature of the surface of the fixing roll above the "hot offset" temperature of the toner particles, which, if present, would cause a splitting action of the toner to result in "hot offset". Splitting occurs when the cohesive forces holding the heated toner particles together are less than the adhesive forces tending to offset it to the contacting surface of the fixing roller. Toner particles can also be offset to the fixing roller by an insufficient application of heat to the toner particles (i.e. "cold" offsetting); by imperfections in the properties of the surface of the fixing roll or by the toner particles insufficiently adhering to the receptor sheet by the forces which normally hold them to the receptor sheet. In such a case, toner particles may be transferred to the surface of the fixing roller with subsequent transfer to the backup roller during periods of time when a receptor sheet is not in the nip presented between the rollers.
One arrangement for minimizing the "offsetting" problem utilizes a fixing roller with an outer surface or covering of a material that is resistant to the high temperatures involved and will accept an offset preventive liquid, such as silicone oil, which is applied to the outer surface of the fixing roller. The silicone oil forms an interface between the fixing roller surface and the toner images carried on the receptor sheet. The relatively low surface energy of the silicone oil, which is presented to the toner, prevents the toner particles from offsetting to the fixing roller surface. Even with the use of the special coverings for the fixing roller and the application of silicone oil to the covering, some "non-visual offsetting" (i.e. offsetting of very fine particles of toner) does occur. Such "offsetting" causes degradation of the copy and in the case of a heated fixing roller, reduces its operating life.
Another fixing station of the contact type applies pressure to the toner particles to permanently affix the toner particles to the receptor sheet. As in the case of the heated fixing roller, some toner is offset to the fixing roller making contact with the toner particles in a pressure fixing apparatus even though an offset preventive liquid, such as silicone oil, is applied to the outer surface of the fixing roller.
The offsetting problem presented by contact type fixing stations has been solved to a degree by the employment of various oiler/cleaner arrangements for the fixing roller. Wick material supplied with silicone based oil from an oil supply has been employed for applying the silicone based oil to the fixing roller and for cleaning the fixing roller. Another known arrangement uses an oil applicator roller covered with an oil absorbent material that is periodically supplied with oil from an oil supply. Web material, which has been impregnated with silicone oil, has been used which is presented to the fixing roller to provide silicone oil to the outer surface of the fixing roller and also remove any toner material that may be present on the fixing roller.
The prior art arrangements mentioned for cleaning and applying offset preventing liquid to the fixing roller have not been entirely satisfactory. In the case of wick plus oil supply arrangements, an excessive amount of oil can build up in the wick during inactive periods causing too much oil to be applied to the fixing rollers. A wick clogging problem due to the toner removed is also presented when an oil supply plus wick arrangement is used. While this is overcome by the use of an arrangement using an oil impregnated web material, an oil migration problem is presented when a supply roll of oil impregnated web material is used. The oil migration problem is minimized by the use of higher viscosity oils (12,000 centistoke), and a heavier weight web to prevent tearing. Such corrective action, however, reduces the web's effectiveness as a cleaner and oiler for the fuser roller. Finally, the prior known arrangement using an oil supply plus an oil applicator roller covered with an oil absorbing material requires the use of a complicated arrangement for supplying a controlled oil to the covered roller. If the amount of oil supplied to the fixing roll is not carefully controlled, excessive oil will transfer to the image receptor making a copy objectionable.